Weishi Wu

Progress with Light-Emitting Polymers

Light-emitting polymers have been studied intensively as materials for light-emitting diodes (LEDs). Here research efforts toward developing these materials for commercial applications are reviewed. The Figure shows the preferred two-layer device structure for commercial polymer LEDs as well as polyfluorene, one of the polymers discussed.

Mobility enhancement in conjugated polymer field-effect transistors through chain alignment in a liquid-crystalline phase

A method is demonstrated by which liquid-crystalline self-organization in rigid-rod nematic conjugated polymers can be used to control the microstructure of the active semiconducting layer in solution-processed polymer thin-film transistors (TFTs). Enhanced charge carrier mobilities of 0.01–0.02 cm2/V s and good operating stability have been achieved in polyfluorene copolymer TFTs by preparing the polymer in a nematic glassy state and by aligning the polymer chains parallel to the transport direction with the help of an alignment layer. Mobility anisotropies of 5–8 for current flow parallel and perpendicular to the alignment direction have been observed that are of the same order of magnitude as optical dichroic ratios.

High brightness and efficiency blue light-emitting polymer diodes

Efficient blue electroluminescence, peaked at 436 nm, is demonstrated from polymer light-emitting diodes operating at high brightness. A dioctyl-substituted polyfluorene was used as the emissive layer in combination with a polymeric triphenyldiamine hole transport layer. The luminance reaches 600 cd/m2 at a current density of 150 mA/cm2 for a bias voltage of 20 V, corresponding to an efficiency of 0.25 cd/A and a luminosity of 0.04 lm/W. These values are optimized at a critical emissive layer thickness.

Electronic structure and energy band gap of poly (9,9-dioctylfluorene) investigated by photoelectron spectroscopy

The electronic structure of poly (9,9-dioctylfluorene) (PFO) film on a Au-coated Si substrate was investigated by ultraviolet photoelectron spectroscopy (UPS) and x-ray photoelectron spectroscopy (XPS). From the UPS measurement, we obtained the ionization potential (Ip) of the PFO film, Ip=5.60±0.05 eV. From the XPS shake-up peaks of the C1s core level, we estimated the electron energy band gap (Eg) of the film, Eg=3.10±0.10 eV. By comparing the Eg with the optical absorption gap, we found that the value of Eg is closer to the optical absorption maximum than to the optical absorption edge. Therefore, we suggest that the optical absorption maximum may be a better approximation than the optical absorption edge in estimating Eg.

Fluorene-based polymers-preparation and applications

This paper briefly reviews our work on the syntheses and applications of a family of fluorene-based polymers. We have found that fluorene as a monomer offers a number of advantages, the most significant being its ability to impart solubility while maintaining a high degree of delocalization. An improved Pd-catalyzed polymerization procedure has enabled the preparation of a large variety of fluorene homopolymers and copolymers. Unlike PPV and related materials, LED devices with fluorene polymers in a conventional configuration appear to have electrons as the majority carrier and their performance is markedly improved when modified with an appropriate polymeric hole transporting layer. An optimized green-emitting device exhibits very high luminance (10 000 cd m-2) at very low bias (< 7 V) and high efficiency (∼10 lm W-1), attributable in part to the high hole mobility of fluorene-based polymers. Applications of these materials to other electronic devices, such as field effect transistors and photocells, are in progress.

Fluorene homopolymers and copolymers

Abstract We report our progress in the preparation of a variety of high molecular weight polymers based on fluorene ring system. A portfolio of fluorene-related homopolymers and copolymers has been developed through Suzuki coupling of 9,9-disubstituted 2,7-bis-1,3,2-dioxaborolanyl-fluorene with a variety of aromatic dibromides. In the case of fluorene homopolymers, the polyphenylene (PP) main chain provides the mechanical, electrical as well as the electronic properties while the presence of carbon-9 forces the phenyl rings to be in plane resulting in improved conjugation. Additionally, carbon-9 provides a site for polymer property modification without altering effective conjugation. In the case of alternating polymers, the optical and electronic properties of the polymers are tailored through selective incorporation of different aromatic units into the system. These polymers are soluble in common organic solvents and are readily processed into thin films of high quality by spin casting. Unlike light emitting diodes (LEDs) based on poly(phenylenevinylene) (PPV) and related materials which have holes as the majority carriers, devices with fluorene polymers appear to have electrons as the majority carrier and their performance is markedly improved when modified with an appropriate polymeric hole transporting layer. Red and green devices have shown lifetimes exceeding 1000 h at a brightness of 150 cd/m2 with minimal voltage change.

Light-emitting diodes based on fluorene polymers

Abstract This paper describes the development of light emitting diode (LED) technology based on fluorene-containing polymers, prepared through the coupling of 9,9-disubstituted 2,7-bis-1,3,2-dioxaborolanyl-fluorene with a variety of aromatic dibromides. In these polymers the polyphenylene-like backbone provides the mechanical and chemical robustness and the C-9 of fluorene provides a site for physical property modifications without introducing significant torsional strain which would adversely affect conjugation. Polymer optical and electronic properties are tailored through selective incorporation of different aromatic unit into the AB alternating structure. LED devices emitting in blue, green, red and other colors are thus obtained.

Anode modification of polyfluorene-based polymer light-emitting devices

A glycerol-modified poly(3,4-ethylene dioxythiophene) (PEDOT): poly(styrene sulfonate) (PSS) layer was used as an anode buffer layer in polymer light-emitting devices using poly(9,9-dioctylfluorene) (F8) as the emitter. Devices with a configuration of indium tin oxide/PEDOT:PSS (with or without glycerol)/F8/CsF/Al were fabricated. It was found that the glycerol-modified device showed a much larger current density than the unmodified device. At an operating voltage of 6 V, the glycerol-modified device showed a luminance of 1300 Cd/m2 and a current efficiency of 1.7 Cd/A compared to the corresponding values of 500 Cd/m2 and 1.3 Cd/A in the unmodified device. Analysis by ultraviolet spectroscopy suggests that the two devices have the same energy level structure and the performance improvement should not be due to change in the PEDOT/polymer interface. It was further found that incorporating a suitable amount of glycerol into the PEDOT:PSS layer can increase its conductivity by six times. This leads to a bett...

Investigations on the grating dynamics in a fast photorefractive guest–host polymer

Abstract Holographic studies of a new guest–host system based on a highly photoconducting fluorene–triarylamine copolymer are presented. The photorefractive grating dynamics are investigated in detail. We propose a new approach to characterize the temporal behavior of these processes. By performing an inverse Laplace transform analysis using the algorithm CONTIN, we were able to identify several processes and to evaluate the according time constants. We compare this method with a conventional procedure to prove its applicability. Both approaches yield almost identical results for the fast time constant which is down to 1 ms for this material at a writing beam intensity of Iwb=1.44 W/cm2.

Distinct interfaces of poly (9,9-dioctylfluorene-co-benzothiadiazole) with cesium and calcium as observed by photoemission spectroscopy

We report that the green-emitting polymer—poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT), a copolymer of poly(9,9-dioctylfluorene) (F8), formed two distinct interfaces with calcium and cesium, respectively, as observed in photoemission measurements. Ultraviolet photoemission spectroscopy showed that Ca formed a stable interface with F8BT without significantly changing the electronic structure of F8BT, which is in contrast to the Cs/F8BT interface where bipolaron states occurred in the forbidden gap as a result of charge transfer processes. X-ray photoemission spectroscopy revealed that Ca covalently bonded with the sulfur atoms whereas Cs preferably interacted with the nitrogen in the F8BT. The results are useful to account for the undesirable device performance using CsF/Al as a cathode in the F8BT-based polymer light-emitting device. The exposure of Cs/F8BT interface to residual gases at a pressure of 2.0×10−9 mbar for 2 and 12 h, respectively, slightly and largely eliminated the gap states. Depos...